This invention is directed to monolithic ceramic catalyst supports and particularly to supports which contain a high surface area phase incorporated within the ceramic matrix itself.
The conventional ceramic monolithic catalyst consists of a ceramic support with a coating of high surface material upon which the catalyst is actually deposited. In particular, the ceramic support is normally prepared by sintering a mold of clay or other ceramic material at a high temperature to impart density and strength. This procedure normally results in a very small surface area, and consequently the ceramic must be coated with another material having a higher surface area, as well as specific chemical characteristics on which to actually deposit the catalyst. This procedure of depositing a high surface area "wash coat" on the low surface area ceramic wall is disclosed, for example, in U.S. Pat. Nos. 2,742,437 and 3,824,196.
Catalyst supports of this kind suffer from several disadvantages. In service, the supports are exposed to a flow of gases which often contain dusts or particulate matter, which can cause the high surface area coating to flake off the underlying ceramic support. This phenomenon can also occur where the support is exposed to thermal cycling because the wash coat and the underlying ceramic material often have different thermal expansion coefficients. Furthermore, catalysts deposited on the high surface area wash coat are susceptible to poisoning, such as by lead or phosphorous in service in automobile converters, and therefore must be periodically regenerated or replaced.
U.S. Pat. No. 4,294,806 discloses the preparation of monolithic supports by extrusion of an alumina ceramic material into the shape of a honeycomb, calcining the material, and then sintering only the front portion. This procedure is said to make the support more abrasion resistant. However, the bulk of the support remains unsintered, so that even though it retains high surface area, the support would lack high strength. U.S. Pat. No. 4,151,121 discloses the preparation of a catalyst by dispersing zeolite and a high surface area alumina (on which a catalytic metal is supported) in a hydrogel of a porous oxide matrix material (such as alumina, clay, silica-alumina composites, and the like) to form a composite mixture. The composite is spray dried, washed free of salts, and then flash dried. Ths method produces catalyst materials in which the high surface material is embedded within a matrix, and thereby somewhat protected from abrasion or poisoning. However, the method is not suitable for the preparation of catalyst support structures that are in monolithic form, the kind normally used in the services where these problems are most prevalent or most severe. British Pat. No. 1,064,018 discloses tubular catalyst supports prepared by forming a paste of alpha-alumina, active alumina, and hydrargillite (a high surface area alumina trihydrate), extruding the paste to form tubular elements, and firing the elements.
It is an object of the present invention to provide a monolithic support having a high surface area which is not easily abraded and which supports catalysts in a manner that resists poisoning. It is a further object of the invention to provide a monolithic support which has good mechanical properties while retaining the porosity and high surface area necessary for proper catalytic functioning. These and other objects are met by the invention to be described.